Final answer:
The closure property states that the difference of two polynomials is always a polynomial, illustrating that polynomial operations do not result in values outside the set of polynomials.
Step-by-step explanation:
The property of polynomial subtraction that says the difference of two polynomials is always a polynomial is referred to as the closure property.
This property implies that when you subtract any two polynomials, the result is always another polynomial.
The subtraction of polynomials does not take us out of the set of polynomials, which effectively highlights the closure aspect of polynomial operations.
It's important to note that this property holds for polynomial addition and multiplication as well.